Non-destructive, wafer scale method to evaluate defect density in heterogeneous epitaxial layers

ABSTRACT

A semiconductor material stack of, from bottom to top, a first semiconductor material having a first lattice constant and a second semiconductor material having a second lattice constant that may or may not differ from the first lattice constant and is selected from an III-V compound semiconductor and germanium is provided. The second semiconductor material of the semiconductor material stack is then scanned using an atomic force microscope (AFM) operating in a tapping mode to provide an AFM image of the second semiconductor material of the semiconductor material stack. The resultant AFM image is then analyzed and crystal defects at a topmost surface of the second semiconductor material of the semiconductor material stack can be measured.

BACKGROUND

The present application relates to a non-destructive method to determinecrystal defects in a heteroepitaxial semiconductor material layer thatis formed on a surface of a semiconductor substrate.

Characterizing wafer and epitaxy defectivity is a fundamental step infabricating working semiconductor devices and circuits. It is costparamount not to waste resources building LSI or VLSI circuits onsubstrates which will not be high yielding. On silicon and siliconepitaxy, numerous etching methods are employed to ensure only thehighest quality, lowest defect density substrates. Also, processingsteps are available so that if the processing has an issue, thesubstrate could be scrapped and costs (both from processing (i.e.,bandwith) and yield loss) could be recovered. With heteroepitaxialsemiconductor materials such as III-V compound semiconductors andgermanium formed on a Si substrate, the prior art etching methods do notwork. In one embodiment, a heteroepitaxial semiconductor material is asemiconductor material that is formed by epitaxy on a surface of anunderlying semiconductor material, wherein the underlying semiconductormaterial has a lattice constant that differs from the lattice constantof the epitaxially grown semiconductor material. GaAs grown on a Sisubstrate is one example.

The current process to completely understand and characterize defecttypes in substrates or heteroepitaxial semiconductor materials is byusing Plan-view transmission electron microscopy (PV-TEM). PV-TEM can beused to measure defect densities down to approximately 10⁶ to 10⁵defects per square centimeter. Because of the small imaging area,however, lower defect densities cannot be measured reliably by PV-TEM.Moreover, PV-TEM is destructive and cannot be employed as an in-lineprocess control metrology.

In view of the above, there is a need to provide a non-destructivemethod to determine crystal defects in heteroepitaxial semiconductormaterials.

SUMMARY

A semiconductor material stack of, from bottom to top, a firstsemiconductor material having a first lattice constant and a secondsemiconductor material having a second lattice constant that may or maynot differ from the first lattice constant and is selected from an III-Vcompound semiconductor and germanium is provided. The secondsemiconductor material of the semiconductor material stack is thenscanned using an atomic force microscope (AFM) operating in a tappingmode to provide an AFM image of the second semiconductor material of thesemiconductor material stack. The resultant AFM image is then analyzedand crystal defects at a topmost surface of the second semiconductormaterial of the semiconductor material stack can be measured.

In one aspect of the present application, a method of measuring crystaldefects in a semiconductor structure is provided. In one embodiment ofthe present application, the method includes providing a semiconductormaterial stack of, from bottom to top, a first semiconductor materialhaving a first lattice constant and a second semiconductor materialhaving a second lattice constant that may or may not differ from thefirst lattice constant and is selected from an III-V compoundsemiconductor and germanium. Next, a topmost surface of the secondsemiconductor material is scanned using atomic force microscope (AFM)operating in a tapping mode to provide an image of the topmost surfaceof the second semiconductor material. A density of crystal defects thatintersect the topmost surface of the second semiconductor material isthen calculated.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a cross sectional view of an exemplary semiconductor structureincluding a semiconductor material stack of, from bottom to top, a firstsemiconductor material having a first lattice constant and a secondsemiconductor material having a second lattice constant that may or maynot differ from the first lattice constant and is selected from an III-Vcompound semiconductor and germanium that can be employed in accordancewith an embodiment of the present application.

FIG. 2 is a cross sectional view of the exemplary semiconductorstructure of FIG. 1 during scanning of the second semiconductor materialof the semiconductor material stack using an atomic force microscope(AFM) operating in a tapping mode.

FIG. 3 is a cross sectional view of the exemplary semiconductorstructure of FIG. 2 after epitaxially depositing another semiconductormaterial having a third lattice constant that may or may not differ fromthe second lattice constant and is selected from an III-V compoundsemiconductor and germanium on the second semiconductor material.

DESCRIPTION

The present application will now be described in greater detail byreferring to the following discussion and drawings that accompany thepresent application. It is noted that the drawings of the presentapplication are provided for illustrative purposes only and, as such,the drawings are not drawn to scale. It is also noted that like andcorresponding elements are referred to by like reference numerals.

In the following description, numerous specific details are set forth,such as particular structures, components, materials, dimensions,processing steps and techniques, in order to provide an understanding ofthe various embodiments of the present application. However, it will beappreciated by one of ordinary skill in the art that the variousembodiments of the present application may be practiced without thesespecific details. In other instances, well-known structures orprocessing steps have not been described in detail in order to avoidobscuring the present application.

Threading defects (or TDs for short) and stacking faults (or SFs forshort) are recombination centers for solid state devices. The term“threading defects” is used throughout the present application to denoteedge, screw and/or mixed edge/screw components of dislocations thattraverse the thickness of a grown semiconductor layer. The term“stacking faults” is used throughout the present application to denoteplanar defects resulting from the formation and growth of partialdislocations or atomic stacking errors during crystalline growth.

On surfaces comprised of Si and Si-epitaxial layers, it is difficult toobserve any step height associated with the nucleus of these crystallinedefects. It was discovered by the Applicant of the present applicationthat on surfaces of heteroepitaxy III-V compound semiconductor layers orgermanium layers that the aforementioned defect nuclei have a measurablesurface displacement associated therewith which can be measured using anatomic force microscope (AFM) operating in a tapping mode. Notably, byusing an atomic force microscope (AFM) operating in a tapping mode, oneis able to directly image crystalline defects of various types thatintersect at the topmost surface of a heteroepitaxy layer of an III-Vcompound semiconductor or germanium. Thus, the present applicationprovides an etch-free and non-destructive method for determining crystaldefects in a heteroepitaxy layer of an III-V compound semiconductor orgermanium.

Referring first to FIG. 1, there is illustrated an exemplarysemiconductor structure including a semiconductor material stack of,from bottom to top, a first semiconductor material 10 and a secondsemiconductor material 12 that can be employed in accordance with anembodiment of the present application. In the present application, thesecond semiconductor material 12 forms an interface with the firstsemiconductor material 10. In some embodiments of the presentapplication, the semiconductor material stack 10, 12 may form a portionof a photovoltaic device.

In accordance with the present application, the first semiconductormaterial 10 of the semiconductor material stack comprises asemiconductor material having a first lattice constant. Thesemiconductor material that provides the first semiconductor material 10of the semiconductor material stack that can be employed in the presentapplication may include, but is not limited to, Si, Ge, SiGe, SiC,SiGeC, III/V compound semiconductor materials such as, for example,InAs, GaAs, InP, GaP, GaN, AlN, InGaAs, InAlP, and InGaP, or II-VIcompound semiconductors. In some embodiments of the present application,Si is employed as the first semiconductor material 10.

In one embodiment of the present application, the semiconductor materialthat provides the first semiconductor material 10 may be a singlecrystalline semiconductor material. By “single crystalline” it is meant,a material in which the crystal lattice of the entire sample iscontinuous and unbroken to the edges of the sample, with no grainboundaries. In another embodiment of the present application, thesemiconductor material that provides the first semiconductor material 10may be polycrystalline or multicrystalline.

In one embodiment of the present application, the first semiconductormaterial 10 may represent an entirety of, or a topmost portion of, abulk semiconductor substrate. By “bulk” it is meant that thesemiconductor substrate is entirely composed of a semiconductor materialor a multilayered stack of semiconductor materials.

In another embodiment of the present application, the firstsemiconductor material 10 may represent a topmost portion of asemiconductor-on-insulator substrate. In such an embodiment, a buriedinsulator layer and an optional handle substrate are located beneath thefirst semiconductor material 10. When present, the optional handlesubstrate is located beneath the buried insulator layer. In someembodiments, the optional handle substrate may comprise one of thesemiconductor materials mentioned above for the first semiconductormaterial 10. In other embodiments, the optional handle substrate and thefirst semiconductor material 10 comprise a same semiconductor material,e.g., Si or Ge. In another embodiment, the optional handle substratecomprises a different semiconductor material than the firstsemiconductor material 10. In yet other embodiments, the optional handlesubstrate is comprised of a non-semiconductor material such as, forexample, a dielectric material and/or a conductive material.

In some embodiments, the first semiconductor material 10 may be anintrinsic semiconductor material (i.e., a semiconductor material that isentirely dopant free or a semiconductor material having a dopantconcentration of less than 1E17 atoms/cm³). In other embodiments, thefirst semiconductor material 10 is entirely doped or contains at leastone doped region. The dopant that may be present in the firstsemiconductor material 10 may be a p-type dopant or an n-type dopant. Asused throughout the present application, “p-type” refers to the additionof impurities to an intrinsic semiconductor material that createsdeficiencies of valence electrons. In one embodiment, the p-type dopantis present in a concentration ranging from 1E17 atoms/cm³ to 1E19atoms/cm³. In another embodiment, the p-type dopant is present in aconcentration ranging from 1E18 atoms/cm³ to 1E20 atoms/cm³. As usedthroughout the present application, “n-type” refers to the addition ofimpurities that contributes free electrons to an intrinsicsemiconductor. In one embodiment, the n-type dopant is present in aconcentration ranging from 1E17 atoms/cm³ to 1E19 atoms/cm³. In anotherembodiment, the n-type dopant is present in a concentration ranging from1E18 atoms/cm³ to 1E20 atoms/cm³.

In some embodiments of the present application, the semiconductormaterial that provides the first semiconductor material 10 may behydrogenated. In another embodiment of the present application, thesemiconductor material that provides the first semiconductor material 10may be non-hydrogenated.

As stated above, the semiconductor material stack that can be employedin the present application also includes the second semiconductormaterial 12. In one embodiment of the present application, the secondsemiconductor material 12 has a second lattice constant that differsfrom the first lattice constant of the first semiconductor material 10and is selected from an III-V compound semiconductor and germanium. Inanother embodiment of the present application, the second semiconductormaterial 12 has a second lattice constant that is the same as the firstlattice constant of the first semiconductor material 10 and is selectedfrom an III-V compound semiconductor and germanium. In such anembodiment the semiconductor material that provides the secondsemiconductor material 12 is compositionally different from thesemiconductor material that provides the first semiconductor material.Throughout the present application, the term “III-V compoundsemiconductor” denotes a semiconductor material that contains at leastone element from Group III of the Periodic Table of Elements and atleast one element from Group V of the Periodic Table of Elements. TheIII-V compound semiconductors may be binary compounds, tertiarycompounds, etc. Examples of III-V compound semiconductors that can beemployed as the second semiconductor material 12 include, but are notlimited to, InAs, GaAs, InP, GaP, InGaAs, InAlP, and InGaP. The atomicpercentage of the Group III element(s) and the atomic percentage of theGroup V element(s) may vary over a wide range. In one embodiment, thefirst semiconductor material 10 is Si, and the second semiconductormaterial 12 is an III-V compound semiconductor material. In anotherembodiment, the first semiconductor material 10 is an III-Vsemiconductor compound, and the second semiconductor material 12 iseither germanium, or an III-V compound semiconductor that iscompositional different from the III-V compound semiconductor of thefirst semiconductor material 10.

In one embodiment of the present application, the semiconductor materialthat provides the second semiconductor material 12 may be a singlecrystalline semiconductor material. In another embodiment of the presentapplication, the semiconductor material that provides the secondsemiconductor material 12 may be polycrystalline or multicrystalline. Insome embodiments of the present application, the semiconductor materialthat provides the second semiconductor material 12 may be hydrogenated.In another embodiment of the present application, the semiconductormaterial that provides the second semiconductor material 12 may benon-hydrogenated.

The second semiconductor material 12 may be an intrinsic semiconductormaterial or it can be doped with either an n-type dopant (as definedabove) or a p-type dopant (as defined above). In some embodiments, thesecond semiconductor material 12 comprises a same conductivity type(either n-type or p-type) as that of the underlying first semiconductormaterial 10. In yet another embodiment of the present application, thesecond semiconductor material 12 has an opposite conductivity type(either n-type or p-type) compared to that of the underlying firstsemiconductor material 10. Thus, the semiconductor material stack of thepresent invention forms a semiconductor material junction that isintrinsic/intrinsic, intrinsic/non-intrinsic, non-intrinsic/intrinsic ornon-intrinsic/non-intrinsic. In embodiments in which anon-intrinsic/non-intrinsic semiconductor material junction is provided,the semiconductor material junction may be one of n-type/n-type,p-type/n-type, n-type/p-type or p-type/p-type.

The second semiconductor material 12 is formed on the firstsemiconductor material 10 utilizing an epitaxial growth (i.e., epitaxialdeposition) process. The terms “epitaxial growth and/or deposition” and“epitaxially formed and/or grown” mean the growth of a semiconductormaterial on a deposition surface of a semiconductor material, in whichthe semiconductor material being grown has the same crystallinecharacteristics as the semiconductor material of the deposition surface.In an epitaxial deposition process, the chemical reactants provided bythe source gases are controlled and the system parameters are set sothat the depositing atoms arrive at the deposition surface of thesemiconductor substrate with sufficient energy to move around on thesurface and orient themselves to the crystal arrangement of the atoms ofthe deposition surface. Therefore, an epitaxial semiconductor materialhas the same crystalline characteristics as the deposition surface onwhich it is formed. For example, an epitaxial semiconductor materialdeposited on a {100} crystal surface will take on a {100} orientation.Thus, and in the present application, the second semiconductor material12 has an epitaxial relationship, i.e., same crystal orientation, asthat of the surface of the first semiconductor material 10.

Examples of various epitaxial growth process that are suitable for usein forming the second semiconductor material 12 of the presentapplication include, e.g., metalorgano chemical vapor deposition(MOCVD), rapid thermal chemical vapor deposition (RTCVD), low-energyplasma deposition (LEPD), ultra-high vacuum chemical vapor deposition(UHVCVD), atmospheric pressure chemical vapor deposition (APCVD) ormolecular beam epitaxy (MBE). The temperature for epitaxial depositionprocess for forming the second semiconductor material 12 typicallyranges from 550° C. to 900° C. Although higher temperature typicallyresults in faster deposition, the faster deposition may result incrystal defects and film cracking.

A number of different source gases which are well known to those skilledin the art may be used for the deposition of the second semiconductormaterial 12. Carrier gases like hydrogen, nitrogen, helium and argon canbe used during the epitaxial growth process. In embodiments in which thesecond semiconductor material 12 is hydrogenated, hydrogen can beintroduced during the epitaxial growth process. In some embodiments, nodopant is present during the epitaxial deposition of the secondsemiconductor material 12. In other embodiments, dopants can be presentduring the epitaxial growth of the second semiconductor material 12. Insome embodiments, no dopant is present during the epitaxial growth ofthe second semiconductor material 12, but dopants can be introduced intothe epitaxially grown second semiconductor material 12 by utilizing oneof ion implantation, gas phase doping or out-diffusion of dopants from adopant source material.

The second semiconductor material 12 that is formed on the firstsemiconductor material 10 may have a thickness that is from 5 nm to 5000nm. Other thicknesses that are lesser than or greater than theaforementioned thickness range may also be employed as the thickness ofthe second semiconductor material 12.

Referring now to FIG. 2, there is illustrated the exemplarysemiconductor structure of FIG. 1 during scanning of the secondsemiconductor material 12 of the semiconductor material stack using anatomic force microscope (AFM) operating in a tapping mode. The scanningprocess of the present application can be used to determine/measurecrystal defects that intersect with a topmost surface of the secondsemiconductor material 12 of the semiconductor material stack. By“intersect with a topmost surface of the second semiconductor material12” it is meant crystal defects such as threading defects and/orstacking faults that have at least one surface that is present at asurface of the second semiconductor material 12 that is opposite thesurface of the second semiconductor material 12 that forms an interfacewith the underlying first semiconductor material 10.

The atomic force microscope (AFM) that can be used in the presentapplication includes any conventional AFM that can be used to measurethe topography and surface roughness of a material layer. The atomicforce microscope (AFM) that can be used in the present applicationincludes a plurality of scanning probes. In FIG. 2, a single scanningprobe 14 is shown for illustrative purposes. Each scanning probe 14 isattached to one end of a cantilever 16 (or body) as known in the art.Each scanning probe 14 has a tip 18 having a radius of curvature. In oneembodiment of the present application, the radius of curvature of tip 18is from 1 nm to 10 nm. In another embodiment of the present application,the radius of curvature of the tip 18 is from 1 nm to 3 nm. In yetanother embodiment of the present application, the radius of curvatureof the tip 18 is from 3 nm to 9 nm.

In one embodiment of the present application, the scanning probe 14 andtip 18 can be made of silicon or any another type of semiconductormaterial. In some embodiments, a Si probe and tip can be made byisotropically etching a silicon pillar structure until the requiredsharpness is reached. In some embodiments, the probe tip 18 may be asemiconductor nanowire that is formed on a semiconductor base thatdefines the body of the scanning probe 14.

The atomic force microscope (AFM) that is used in the presentapplication is employed in a tapping mode (i.e., intermittent tipcontact). In such a mode, the cantilever 16 is typically oscillated witha large amplitude from 100 nm to 200 nm. In such a mode, short rangeforces are detectable without sticking to the surface of the secondsemiconductor material 12. The short range forces that are detectableare translated to topography by a sensor (not shown). The sensor can beany conventional sensor used in atomic force microscopy including, forexample, a bulk-component-based free-space laser beam deflection setupwith a four quadrant photodiode acting as the deflection sensor. Anexample of another sensor that can be employed in the presentapplication is a piezoresistive deflection sensor.

In accordance with an embodiment of the present application, thescanning of the topmost surface of the second semiconductor material 12comprises a raster scan. By “raster scan” it is meant that the scanningprobe 14 sweeps horizontally left-to-right (or right-to-left) at asteady rate, then blanks and rapidly moves back to the left (or right),where it turns back on and sweeps out the next line.

In accordance with an embodiment of the present application, scanning isperformed at a frequency from 1 Hz to 1 khz. In another embodiment ofthe present application, scanning is performed at a frequency from 50kHz to 500 kHz. In accordance with yet another embodiment of the presentapplication, scanning is performed at a frequency from 5 Hz to 3 Hz. Inaccordance with an embodiment of the present application, scanning isperformed at a step size from 1 nm to 10 nm. In another embodiment ofthe present application, scanning is performed at a step size from 1 nmto 5 nm. In yet another embodiment of the present application, scanningis performed at a step size from 2 nm to 50 nm.

The atomic force microscope (AFM) that is employed in the presentapplication provides an image (i.e., topography image) of the topmostsurface of the second semiconductor material 12. Defects appear as pointdepressions on the AFM image for the TD defect. For the SF defect theyappear as lines or boxes of low height material on the image. From theimage that is provided, the density of crystal defects that intersectwith the topmost surface of the second semiconductor material 12 can becalculated. The calculating of the density of crystal defects thatintersect with the topmost surface of the second semiconductor material12 can be performed by identifying discrete small spatial resolutionregions (or features) which have signature regions of less height thanthe surrounding material. Next, the discrete identified regions (orfeatures) are totaled and thereafter the areal density of the discreteidentified regions (features) is calculated. The density can be reportedas the number of crystal defects divided by area, in cm², of theanalyzed region.

The atomic force microscope (AFM) that can be used in the presentapplication can measure crystal defects that intersect with the topmostsurface of the second semiconductor material 12 that have a defectdensity that is less (i.e., less than 10⁶ defects per square centimeter)than a conventional Plan-view transmission electron microscopy (PV-TEM).Typically, the atomic force microscope (AFM) that can be used in thepresent application can measure crystal defects that have a defectdensity of from 10⁴ defects per square centimeter to 10¹⁰ defects persquare centimeter.

Referring now to FIG. 3, there is illustrated the exemplarysemiconductor structure of FIG. 2 after epitaxially depositing anothersemiconductor material 20 on the second semiconductor material 12. Theanother semiconductor material 20 has a third lattice constant that mayor may not differ from the second lattice constant and is selected froman III-V compound semiconductor and germanium. The another semiconductormaterial 20 may be single crystalline or multicrystalline. The anothersemiconductor material 20 may be hydrogenated or non-hydrogenated. Theanother semiconductor material 20 can be intrinsic or doped. In oneembodiment and when doped, the another semiconductor material 20 canhave a same conductivity type dopant as the second semiconductormaterial 12. In another embodiment and when doped, the anothersemiconductor material 20 can have an opposite conductivity than thesecond semiconductor material 12. The another semiconductor material 20can be epitaxially deposited as described above in providing the secondsemiconductor material 12. The another semiconductor material 20 canhave a thickness within the range provided above for the secondsemiconductor material.

The another semiconductor material 20 may then be scanned using anatomic force microscope (AFM) as described above to determine thedensity of crystal defects that intersect with the topmost surface ofthe another semiconductor material 20.

Additional semiconductor materials (not shown) can be formed byepitaxial deposition one atop the other. Each additional semiconductormaterial has a lattice constant that may or may not differ from theprevious lattice constant and is selected from an III-V compoundsemiconductor and germanium. The density of crystal defects thatintersect with the topmost surface of a particular semiconductormaterial can be determined as described above.

While the present application has been particularly shown and describedwith respect to various embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formsand details may be made without departing from the spirit and scope ofthe present application. It is therefore intended that the presentapplication not be limited to the exact forms and details described andillustrated, but fall within the scope of the appended claims.

What is claimed as new is:
 1. A non-destructive method of measuringcrystal defects in a semiconductor structure, said method comprising:providing a semiconductor material stack of, from bottom to top, a firstsemiconductor material having a first lattice constant and a secondsemiconductor material having a second lattice constant that may or maynot differ from said first lattice constant and is selected from anIII-V compound semiconductor and germanium; scanning a topmost surfaceof said second semiconductor material using atomic force microscope(AFM) operating in a tapping mode to measure crystal defects thatintersect said topmost surface of said second semiconductor material andto provide an image of said topmost surface of said second semiconductormaterial; and calculating a density of said crystal defects thatintersect said topmost surface of said second semiconductor material. 2.The non-destructive method of claim 1, wherein said providing saidsemiconductor material stack comprises: epitaxially depositing saidsecond semiconductor material on said first semiconductor material. 3.The non-destructive method of claim 1, wherein said first semiconductormaterial is silicon and said second semiconductor material is an III-Vcompound semiconductor.
 4. The non-destructive method of claim 3,wherein said III-V compound semiconductor contains an n-type dopant. 5.The non-destructive method of claim 3, wherein said III-V compoundsemiconductor contains a p-type dopant.
 6. The non-destructive method ofclaim 1, wherein said first semiconductor material is silicon and saidsecond semiconductor material is germanium.
 7. The non-destructivemethod of claim 6, wherein said germanium contains an n-type dopant. 8.The non-destructive method of claim 6, wherein said germanium contains ap-type dopant.
 9. The non-destructive method of claim 1, wherein saidcrystal defects are selected from stacking faults, threading defects andcombinations thereof.
 10. The non-destructive method of claim 1, whereinsaid atomic force microscope (AFM) comprises a plurality of scanningprobes, each scanning probe is attached to one end of a cantilever. 11.The non-destructive method of claim 10, wherein each scanning probe hasa tip having a radius of curvature.
 12. The non-destructive method ofclaim 11, wherein said radius of curvature is from 1 nm to 10 nm. 13.The non-destructive method of claim 1, wherein said scanning comprises araster scan.
 14. The non-destructive method of claim 1, wherein saidscanning is performed at a frequency from 1 Hz to 1 kHz.
 15. Thenon-destructive method of claim 1, wherein said scanning is performed ata step size from 1 nm to 10 nm.
 16. The non-destructive method of claim1, wherein said tapping mode comprises intermittent contact of a probetip on said topmost surface of said second semiconductor material. 17.The non-destructive method of claim 1, wherein said calculating saiddensity of said crystal defects comprises totaling of discreteidentified features in an image and calculating an areal density of saidfeatures.
 18. The non-destructive method of claim 1, wherein saidtopmost surface of said second semiconductor material is not subjectedto etching prior to said scanning.
 19. The non-destructive method ofclaim 1, further comprising epitaxially depositing another semiconductormaterial having a third lattice constant that may or may not differsfrom said second lattice constant and is selected from an III-V compoundsemiconductor and germanium on said second semiconductor material andafter said scanning and said calculating.
 20. The non-destructive methodof claim 19, further comprising measuring crystal defects in saidanother semiconductor material, wherein said measuring said crystaldefects comprises: scanning a topmost surface of said anothersemiconductor material using atomic force microscope (AFM) operating ina tapping mode to provide an image of said topmost surface of saidanother semiconductor material; and calculating a density of crystaldefects that intersect said topmost surface of said anothersemiconductor material.